OBJECTIVE: To examine the association between prepregnancy body mass index (BMI) and neonatal mortality while accounting for the timing of delivery and subtypes of preterm birth. METHODS: The study population included 85,375 liveborn singletons of mothers in the Danish National Birth Cohort (1996-2002) who were interviewed during the second trimester. Information about pregnancy outcomes and neonatal deaths (n=230) was obtained from national registers. The association was estimated by Cox regression analyses and results were presented as hazard ratios with 95% confidence intervals (CIs). RESULTS: Compared with infants of mothers who were at a normal weight before pregnancy (BMI of 18.5 or more but less than 25), neonatal mortality was increased in infants of mothers who were overweight (BMI of 25 or more but less than 30) or obese (BMI of 30 or more) (adjusted hazard ratios 1.7, CI 1.2-2.5, and 1.6, CI 1.0-2.4, respectively). For preterm infants (n=3,934, 136 deaths), neonatal mortality in infants born after preterm premature rupture of membranes (PROM) was significantly increased if they were born to an overweight or obese mother (adjusted hazard ratios 3.5, CI 1.4-8.7, and 5.7, CI 2.2-14.8). There were no associations between high BMI and neonatal mortality in infants born after spontaneous preterm birth without preterm PROM or in infants born after induced preterm delivery. CONCLUSION: High maternal weight seems to increase the risk of neonatal mortality, especially in infants born after preterm PROM. Inflammation or infection related to obesity may be part of the causal pathway. LEVEL OF EVIDENCE: II. Udgivelsesdato: 2007-Nov